The German AS 20 60 884 describes a test device for indicating the polarity and the phase state of electric potentials relative to a given reference potential, wherein two luminescent diodes connected in antiparallel fashion with a preceding current limiting resistor are employed. The two luminescent diodes are arranged in protected fashion in at least one opening in a pencil-like housing which simultaneously contains a current limiting resistor and at its end side bears a metal point. The walls of the pencil-like housing possess an opening through which the light output of two luminescent diodes can be viewed externally. The two luminescent diodes are connected in antiparallel fashion and connected on the one hand via the current limiting resistor to the metal point and on the other hand to a test line which leads out of the pencil-like housing. In the testing of a d.c. voltage, in accordance with the polarity, one or the other luminescent diode is illuminated whereas in the testing of an a.c. voltage both luminescent diodes are illuminated.
If the current limiting resistor is in the form of a ceramic cold conductor which--as is known--consists of ferro-electric material on a base of n- and/or p-doped barium titanate and in the region of the Curie temperature exhibits a sudden increase in its electrical resistance by approximately 4 powers of ten, this device can be used to check voltages from approximately 2 V and up to 500 V. The ceramic cold conductor exerts a protective function upon the luminescent diodes in that it limits the current flow by virtue of self heating above the Curie temperature and an associated considerable increase in resistance.
However, in many cases it is not only of interest whether a device is connected to current or whether a voltage is present and the nature of the polarity thereof, but in many cases it is also to be confirmed whether an electric conductor, an electric component, an electric assembly and the like is traversed by current. This checking of the passage of current and eventually e.g. by semiconductor devices the direction of current flow (continuity) is therefore of particular interest.
Current passage checking devices operate in accordance with the principle that a current is conducted from a voltage source through the electrical conductor which is to be checked. An acoustic or optical signal then indicates whether there is a passage of current or not.
The U.S. Pat. No. 3,870,950 describes an input protection circuit for electric current passage checking device which contains a separate current source and a current flowing indicator for testing voltage-free circuits and for protection from alien voltages which could jeopardize the current flow indicator, a semiconductor switch and a cold-conducting resistor connected in series, where the free ends represent the input terminals and the current source and the current flow indicator are connected in series and the free ends of this series arrangement are connected on the one hand to the free end of the semiconductor switch and on the other hand to the connection point between semiconductor switch and cold conducting resistor.